setenv fix memory leak when setting large, duplicate string (BZ #17658)

glibc maintains a binary tree of environment strings it malloc()ed
itself.  However, it's possible for it to malloc() a string, then find
that an identical string is already in the tree.  In this case, the
memory is leaked and is not freed if the application later calls
__libc_freeres().  Fix this by freeing 'new_value' when it's unneeded.

Test case:
	#include <stdlib.h>
	#include <string.h>

	int main()
	{
		char *p = calloc(100000, 1);
		memset(p, 'A', 99999);
		setenv("TESTVAR", p, 1);
		setenv("TESTVAR", p, 1);
		free(p);
	}

Leak that was reported by valgrind:
	100,008 bytes in 1 blocks are definitely lost in loss record 1 of 1
	   at 0x4C29F90: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
	   by 0x4E6B3D4: __add_to_environ (setenv.c:176)
	   by 0x4C31B8F: setenv (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
	   by 0x400642: main (in /mnt/tmpfs/a.out)

1 files changed, 4 insertions, 4 deletions

diff --git a/NEWS b/NEWS
index 8582885..bec70dc 100644
--- a/NEWS
+++ b/NEWS
@@ -14,10 +14,10 @@ Version 2.21
   17273, 17344, 17363, 17370, 17371, 17411, 17460, 17475, 17485, 17501,
   17506, 17508, 17522, 17555, 17570, 17571, 17572, 17573, 17574, 17581,
   17582, 17583, 17584, 17585, 17589, 17594, 17601, 17608, 17616, 17625,
-  17630, 17633, 17634, 17635, 17647, 17653, 17657, 17664, 17665, 17668,
-  17682, 17717, 17719, 17722, 17723, 17724, 17725, 17732, 17733, 17744,
-  17745, 17746, 17747, 17775, 17777, 17780, 17781, 17782, 17793, 17796,
-  17797, 17806
+  17630, 17633, 17634, 17635, 17647, 17653, 17657, 17658, 17664, 17665,
+  17668, 17682, 17717, 17719, 17722, 17723, 17724, 17725, 17732, 17733,
+  17744, 17745, 17746, 17747, 17775, 17777, 17780, 17781, 17782, 17793,
+  17796, 17797, 17806
 
 * i386 memcpy functions optimized with SSE2 unaligned load/store.